Process of regenerating a high-boiling scrubbing solution which contains CO2 and H2 S

ABSTRACT

A scrubbing solution boiling above 100° C. under atmospheric pressure is used for removing H 2  S and CO 2  from a gas stream at a temperature from 5° to 70° C. The solution is thus laden with at least CO 2  and H 2  S. For regeneration, the laden scrubbing solution under a pressure of at least 2 bars and at a temperature of at least 60° C. is supplied to an expansion zone and is expanded therein. A flashed-off gas which is rich in H 2  S is withdrawn from the expansion zone. The scrubbing solution from the expansion zone is subjected to a hot regeneration, by which the scrubbing solution at a temperature below its boiling temperature is stripped with an inert gas so that its residual loading is substantially removed. Mixed gases which contain inert gas are withdrawn from the hot regeneration and are contacted with scrubbing solution.

DESCRIPTION

This invention relates to a process of regenerating a high-boilingscrubbing solution which has been used for gas purification and has aboiling point above 100° C. under atmospheric pressure and has beenladen at least with CO₂ and H₂ S in gas purification at temperaturesfrom 5° to 70° C. and the regenerated scrubbing solution is re-used forgas purification.

The main components of the high-boiling scrubbing solutions to beemployed consist, e.g., of N-methylpyrrolidone (NMP) or dimethyl etherof polyethylene glycol (DMPEG). The water content of the scrubbingsolutions is relatively low and in most cases below 10% by weight.

In the process of purifying a gas stream which is described in EuropeanPatent 0 054 772 the low-boiling scrubbing solution which is laden withCO₂ and H₂ S is fed through an expansion zone to a re-absorption zoneand subsequently to the hot regeneration zone. In the re-absorptionzone, part of the CO₂ is removed overhead and a gas which is rich in H₂S is withdrawn from the hot regeneration zone and is fed to a Clausprocess plant. The hot-regenerated scrubbing solution contains methanolas its main constituent and is recycled to the gas scrubber. Buthigh-boiling scrubbing solutions cannot be regenerated as easily aslow-boiling scrubbing solutions, such as methanol.

It is an object of the invention to regenerate high-boiling scrubbingsolutions in a simple and effective manner and without heating of thescrubbing solution to its boiling point. In the process described firsthereinbefore this is accomplished in accordance with the invention inthat the laden scrubbing solution having a pressure of at least 2 barsand a temperature of at least 60° C. is expanded into an expansion zone,a flashed-off gas which is rich in H₂ S is withdrawn from the expansionzone, the scrubbing solution coming from the expansion zone is subjectedto hot regeneration, wherein the scrubbing solution at a temperaturebelow its boiling temperature is stripped with an inert gas tosubstantially remove the residual loading of the scrubbing solution, andthe inert gas-containing mixed gases which have been withdrawn from thehot regeneration are contacted with scrubbing solution.

In the process according to the invention it is readily possible towithdraw from the expansion zone a flashed-off gas which contains atleast 50% by volume H₂ S and preferably contains in excess of 70% byvolume H₂ S and such flashed-off gas may be fed to a Claus processplant. In the Claus process plant a mixture of H₂ S and SO₂ is producedby partial combustion in a manner known per se and is reacted to produceelementary sulfur and water.

In accordance with a further feature of the process, the laden scrubbingsolution coming from the gas purification is initially fed to are-absorption zone, which is also fed with the inert gas-containingmixed gases from the hot regeneration zone, and the scrubbing solutionis subsequently fed to the expansion zone. In the re-absorption zone,mainly CO₂ is removed from the laden scrubbing solution so that theflashed-off gas which comes from the expansion zone has only a lowcontent of residual CO₂ and for this reason can be more easily processedin a Claus process plant.

BRIEF DESCRIPTION OF THE DRAWINGS

Possible embodiments of the process will be explained with reference tothe drawing, in which

FIG. 1 is a flow scheme of a first embodiment of the process and

FIG. 2 is a flow scheme of a second embodiment of the process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The gas which is to be purified contains CO₂ and H₂ S and is fed in line1 to the lower portion of a gas purifier 2, which usually consists of ascrubbing column. High-boiling scrubbing solution is fed to the gaspurifier 2 through line 3 and the purified gas is withdrawn in line 4.The gas to be purified may consist, e.g., of raw synthesis gas or ofnatural gas, although the process is not restricted to the treatment ofsuch gases. In the gas purifier 2 the temperatures are in the range from5° to 70° C. and the pressure is in most cases in the range from 5 to100 bars.

Scrubbing solution laden with CO₂ and H₂ S is withdrawn in line 6 and ispreferably first fed to a re-absorption zone 7. Regenerated scrubbingsolution is fed via line 8 to the top of the re-absorption zone. Mixedgases which contain an inert gas and are rich in H₂ S are fed via line 9to the lower portion of the zone 7. In the re-absorption zone 7, thosemixed gases act mainly as a stripping gas for removing CO₂ from thescrubbing solution which is fed in line 6. Owing to the regeneratedscrubbing solution fed via line 8, the mixed gases withdrawn in line 10are virtually free of H₂ S and for this reason may be discharged intothe atmosphere or combined with the scrubbed gas in line 4.

The scrubbing solution leaves the re-absorption zone 7 through line 12and is fed by the pump 13 first through a heat exchanger 14 and isheated up therein and then enters through an expansion valve 15 anexpansion vessel 16. Because the scrubbing solution is expanded throughthe expansion valve 15 into the vessel 16, a flashed-off gas isreleased, which is rich in H₂ S and is present in line 18 under apressure of 1 to 6 bars and can be supplied to a Claus process plant,not shown. In the Clause process plant, elementary sulfur can berecovered in a manner known per se. For the Claus process the gas to beprocessed should contain H₂ S in the highest possible concentration. Theflashed-off gas withdrawn in line 18 usually contains at least 50% byvolume, in most cases at least 70% by volume H₂ S. For this reason partof the H₂ S content has been removed from the scrubbing solution whichis withdrawn in line 19 from the expansion vessel 16.

For a complete regeneration, the scrubbing solution is fed in line 19 toa hot regeneration zone 20, in which the scrubbing solution is heatedand stripped with an inert gas to remove residual H₂ S and CO₂. Heatingis effected in a manner known per se by a heater 21. For a simultaneousstripping, inert gas is fed via line 23 to the hot regeneration zone 20.That stripping gas may consist, e.g., of nitrogen or of a partial streamof the pure gas in line 4.

Mixed gases, which contain inert gas, are withdrawn in line 25 from thehot regeneration zone 20 and are passed through a cooler 26 and are thenfed in line 9 to the re-absorption column 7. Regenerated scrubbingsolution flows in line 27 through the pump 28 first to the heatexchanger 14 and is cooled therein to temperatures not in excess of 70°C. The scrubbing solution is then supplied through a further cooler 29and through line 30 to flow in part through line 8 to the re-absorber 7whereas a major part is supplied in line 3 to the gas purifier 2. There-absorption column 7 and the hot regeneration zone 20 contain packingmaterial known per se or plates which are permeable to liquid so thatthe mass transfer will be increased.

The embodiment of the process which is illustrated in FIG. 2 differsfrom the process of FIG. 1 in that the re-absorber 7a does not have atop portion that is supplied with regenerated scrubbing solution. As aresult, the exhaust gas which is withdrawn in line 35 still has acertain H₂ S content and is fed by the fan 36 to the gas purifier 2. Theremaining details of the process illustrated in FIG. 2 are the same asin the process of FIG. 1; this is indicated by the use of identicalreference numerals.

The re-absorber 7a may be entirely omitted. In that case the line 9 inthe process system of FIG. 2 is connected to the fan 36 and the line 6is connected to the pump 13. But the re-absorber 7 shown in FIG. 1 or 7ashown in FIG. 2 has the advantage that a flashed-off gas which is richin H₂ S and has only a low CO₂ content can be withdrawn without a needfor expensive equipment.

EXAMPLE

In a process as illustrated in FIG. 2, the gas purifier 2 may besupplied through line 1 at a rate of 10,000 kilomoles per hour withmixed gases consisiting of a valuable gas, CO₂ and H₂ S and said mixedgases may be scrubbed with 300 m³ NMP scrubbing solution per hour. Thiswill result in the various lines in the calculated gas rates stated inthe Table.

    ______________________________________                                        Line  1      4       6    9    12   18  23   25   35                          ______________________________________                                        Valu- 8260   8260     18  --   --   --  --   --    18                         able                                                                          gas                                                                           CO.sub.2                                                                            1650   1648.5  205  --    1.5 1.5 --   --   203.5                       H.sub.2 S                                                                            90    --      133   57  147  90  --    57   43                         N.sub.2                                                                             --      259.5  --   260   0.5 0.5 260  260  259.5                       ______________________________________                                    

All rates are calculated in kilomoles per hour. The valuable gas mayconsist, e.g., of hydrogen or methane or of a mixture of H₂ and CO.

The pressures and temperatures are as follows:

    ______________________________________                                                          Pressure                                                                             Temperature                                          ______________________________________                                        In the gas purifier 2                                                                              30 bars 25° C.                                    in line 35          1.1 bars 25° C.                                    in line 9           1.2 bars 25° C.                                    before the expansion valve 15                                                                       5 bars 110° C.                                   in line 18          2.4 bars 25° C.                                    in the hot regeneration zone 20                                                                   1.6 bars 120° C.                                   ______________________________________                                    

After the hot regeneration effected by heating and stripping, thescrubbing solution in line 27 is at a temperature of 120° C. and issupplied at 20° C. in line 3 to the gas purifier 2.

It will be understood that the specification and examples areillustrative but not limitative of the present invention and that otherembodiments within the spirit and scope of the invention will suggestthemselves to those skilled in the art.

What is claimed is:
 1. In the purification of a gas containing CO₂ andH₂ S wherein the gas is purified at a temperature from about 5° C. to70° C. with a scrubbing solution having a boiling point above about 100°C. at atmospheric pressure to produce a purified gas and a spentscrubbing solution, said spent scrubbing solution being treated in are-absorption zone and mixed gases are withdrawn from said zone, thespent scrubbing solution withdrawn from said zone being fed to anexpansion zone, the solution fed to the expansion zone having atemperature of at least about 60° C. and a pressure of at least 2 bars,expanding the scrubbing solution into the expansion zone thereby toflash off a gas which is rich in H₂ S, withdrawing the flashed-off gasfrom the expansion zone, subjecting the scrubbing solution coming fromthe expansion zone to hot regeneration by stripping the scrubbingsolution at a temperature below its boiling temperature with an inertgas substantially to remove the residual loading of the scrubbingsolution, withdrawing regenerated scrubbing solution from saidregeneration and recycling at least a portion thereof to saidpurification, withdrawing inert gas-containing mixed gases from saidregeneration and feeding said mixed gases into said re-absorption zone.2. A process according to claim 1, wherein the flashed-off gas which isrich in H₂ S is under a pressure of 1 to 6 bars in the expansion zone.3. A process according to claim 1, wherein the flashed-off gas withdrawnfrom the expansion zone contains at least about 50% by volume H₂ S.
 4. Aprocess according to claim 1, wherein a partial stream of thehot-regenerated scrubbing solution is fed to the re-absorption zone. 5.A process according to claim 1, wherein exhaust gas withdrawn from there-absorption zone is fed to the initial gas purification.
 6. A processaccording to claim 1, wherein the flashed-off gas which is rich in H₂ Sis fed to a Claus process plant for producing elementary sulfur.
 7. Inthe purification of a gas containing CO₂ and H₂ S wherein the gas ispurified at a temperature from about 5° C. to 70° C. with a scrubbingsolution having a boiling point above about 100° C. at atmosphericpressure to produce a purified gas and a spent scrubbing solution, saidspent scrubbing solution being treated in a re-absorption zone and mixedgases are withdrawn from said zone, the spent scrubbing solutionwithdrawn from said zone being fed to an expansion zone, the solutionfed to the expansion zone having a temperature of at least about 60° C.and a pressure of at least 2 bars, expanding the scrubbing solution intothe expansion zone thereby to flash off a gas which is rich in H₂ S,withdrawing the flashed-off gas from the expansion zone, subjecting thescrubbing solution coming from the expansion zone to hot regeneration bystripping the scrubbing solution at a temperature below its boilingtemperature with an inert gas substantially to remove the residualloading of the scrubbing solution, withdrawing regenerated scrubbingsolution from said regeneration and recycling at least a portion thereofto said purification, withdrawing inert gas-containing mixed gases fromsaid regeneration and feeding said mixed gases into the initial gaspurification zone.